The normal cardiac cycle includes contractions of the atrial muscles, which are activated by the autonomic sinoatrial node (SA node), also called the sinus node. The electrophysiologic (EP) signal generated by the SA node spreads in the right and left atrium leading to their contraction. The EP signal further reaches the atrioventricular node (AV node) situated between the atria and the ventricles. The AV node delays the EP signal, giving the atria time to contract completely before the ventricles are stimulated. After the delay in the AV node the EP signal spreads to the ventricles via the fibers of the His-Purkinje system leading to the contraction of the ventricles. After the contraction the atria are relaxed and filled by blood coming from venous return. The entire cardiac cycle is the combination of atrial and ventricular contraction, i.e. depolarization, and their relaxation, i.e. repolarization.
As is known, the cardiac cycle can be measured non-invasively by attaching small electrodes on the skin of the patient. The voltage differences caused by the heart between the electrodes are measured and recorded in order to obtain the electrocardiogram (ECG) of the patient.
In this connection, reference is made to FIG. 1, which shows one cycle of an ECG signal. As is commonly known, and also shown in the figure, the waves of the ECG signal (i.e. the depolarisation and repolarisation events in the heart) are named alphabetically from P to U. The ECG signal shows each phase of the cardiac cycle: the P wave represents the systole of the atria, the QRS wave represents the systole of ventricles, and T wave represents the repolarization of the ventricles. Modern ECG devices use digital signal processing to analyze the shape and the consistency of, and the durations between these waveforms.
The heart rate (HR) can be measured by calculating the number of QRS waves in a minute. The HR may be expressed as a minute rate or as beats per minute (bpm). The rate of a heart functioning in a normal manner is not a constant, and the variation of the rate, which is commonly called the heart rate variability (HRV), has become one of the widely used markers for indicating the cardiac condition of a patient.
The ECG signal is thus analyzed for detecting various heart disorders, such as abnormalities in the heart rhythm, also termed arrhythmias. One of type of arrhythmia is an AV block, also termed a heart block, in which the signal flow from the atria to the ventricles is impaired. The AV blocks are classified into three different categories according to the level of impairment. In a first-degree AV block, the electrical impulse travels through the AV node more slowly than normal. If the PR interval measured from the ECG signal is more than about 0.2 seconds but less than about 0.6 seconds, it is regarded as the first-degree AV block. Normally, the first-degree AV block does not need treatment, but requires careful monitoring since it may progress to a more serious type of AV block. However, a difficulty in documenting a first-degree AV block is that such blocks are sporadic and may therefore not appear during a clinical visit. Instead, they may appear during the night time at home. In a second-degree AV block, some impulses from the atria do not reach the ventricles. In this case, the relevant P wave is not followed by a QRS complex, since the ventricles are not activated. FIG. 2 shows an ECG signal in which P wave P(n), i.e. the P wave of the cardiac cycle with index n, is not followed by a QRS complex. There are two types of second-degree AV blocks; type I and type II. Both types may progress to a third-degree AV block. However, as type II second-degree block may do that rapidly, it is more serious than type I. In a third-degree AV block, the electrical impulses from the atria do not reach the ventricles, i.e. there is a total block of atrial impulses to the ventricles. The third-degree AV block involves a serious risk of sudden cardiac death and therefore requires a pacemaker to be implanted immediately. A temporary pacemaker may also be used until a surgery can be performed.
At present, the detection of AV blocks is commonly based on the ECG interpretation performed by physicians. As this cannot normally be carried out in real-time, it is difficult to get a real-time indication of the onset of an AV block or of a rapid progress of the AV block. Furthermore, documentation of sporadically occurring AV blocks is difficult, since it requires ECG signal data to be collected over a long time period.
A further difficulty related to the monitoring of atrial activity is that Bundle Branch Blocks (BBBs), for example, may suddenly cause a substantial drop in the amplitude of the QRS complexes. The detection of these low amplitude QRS complexes may be complicated, since the improved detection sensitivity required by the lower amplitudes tends to translate to an increased number of erroneous detections caused by artifacts, for example.
The present invention seeks to provide a mechanism for eliminating or alleviating the above drawbacks related to the monitoring of atrial activation.